1. Field of the Invention
This invention relates to jam resistant command and control data links. More particularly, the apparatus and system are employed to decrease the acquisition time for data links which employ spread spectrum codes.
2. Description of the Prior Art
Heretofore, different methods have been discussed for providing jam resistant control data links. A method of providing jam resistant transmission of data is to employ spread spectrum coding of the data to be transmitted. One such method of avoiding jamming is to employ direct sequence modulation, wherein a binary code with noise-like qualities is modulated on to a carrier. The most desirable selection of a secure code requires a long sequence and a long sequence requires long acquisition time.
The demodulation of the incoming modulated waveform is dependant on having a replica of the sequence being transmitted which is in phase with the incoming sequence. The reason acquisition time is lengthy is that the incoming sequence must be phase searched until correlation is attained. Phase searching extends acquisition time and has heretofore been a drawback in the use of desirable spread spectrum codes.
The process of phase searching can be speeded up using tapped delay lines or matched filters, but this greatly increases the cost and the complexity of the decoding equipment. It has been suggested that the matched filtering can be performed at high data rates with surface acoustic wave devices (SAWs). It has been suggested that a plurality of SAWs be provided, one for each sequence group of signals representative of a symbol being transmitted. If all such SAWs receive the incoming signals simultaneously, then the output of the SAWs could be compared in a complex computer system and the SAW with the greatest output voltage would be indicative of the symbol or signal being transmitted. Numerous problems arise from using the same number of matched filters as there are symbols being transmitted. Since a practical SAW is presently only capable of storing ten to twenty microseconds of waveform signal, there are limitations to the data rate of transmission. When SAWs are employed in low data rate systems, they are only capable of collecting a portion of the signal energy defining the data bit being transmitted. For example, when data is being transmitted at 4.88 kilobits per second, the time for one bit is 205 microseconds. The SAW is only capable of utilizing or collecting ten to twenty microseconds of the available energy of this bit. Thus, the SAW is only recovering one tenth to one twentieth of the available signal energy. It is not practical to connect SAWs in series because the attenuation factor is so great that the efficient recovery of energy is not possible. Signal attenuation limits a practical SAW to a time length of about fifteen microseconds. However, it will be understood that conventional tapped delay lines could be employed to create delays greater than fifteen microseconds by using semi-conductor devices such as integrated circuits and/or charged coupled devices. Semiconductor delays would require more complex devices than those accomplished with a SAW. Inductive delays are not considered to be practical or accurate enough for consideration.
In order to overcome high energy jamming, which is partially effective against spread spectrum codes, it is most desirable to collect all of the energy available at the receiver. The equivalent of collecting only ten to fifteen percent of the energy available is to effectively increase the efficiency of the jammer by a factor of seven to ten.
Typical data rates for controlling a single remote piloted vehicle (RPV) are two hundred bits per second so that the data rate to control twenty RPVs from a single master station would be on the order of four thousand bits per second. A data rate of four thousand bits per second is equivalent to a bit time of two hundred and fifty microseconds, thus, the SAW device of the prior art would only recover a small portion of the energy transmitted and would permit the jammers to be more effective. In order to employ real time control with a plurality of RPVs, when employing spread spectrum codes, the code stream must be reacquired each time a new RPV is addressed. Reacquisition time could be extremely lengthy and difficult when using the prior art methods of sequential scanning.
It would be extremely desirable to employ a surface acoustic wave device in a spread spectrum code recovery system for RPVs which would permit rapid reacquisition of the signal with little or no loss of energy.